Currently, you can use the "auto-size content" options under App Properties > Size to create an app that will automatically fit itself to different screen sizes. This works well for apps designed for devices with roughly similar resolutions. You might want to create two versions of the app - one for phone and one for tablets - but you don't have to.

There may be alternative dynamic sizing options in the future, but the concept is difficult to simplify for a WYSIWYG type interface.